The Science of the Scoop: How UnderTrays Save Semis 10% on Fuel

Ever driven behind a semi and noticed that weird scoop-shaped panel under the trailer? Turns out, it’s not random.

That scoop is part of an aerodynamic setup designed to cut down on drag—and save fuel. At highway speeds, big rigs push an insane amount of air out of the way (imagine a moving wall hitting 18 tons of air per mile), and all that resistance burns fuel fast.

To help, companies like SmartTruck created the UnderTray—basically a curved panel that sits underneath the trailer and guides airflow in a smarter way. Instead of air just smashing into the axles or swirling uselessly at the back, the scoop redirects it, helping fill the low-pressure pocket behind the truck. Less drag means better mileage. SmartTruck claims up to a 10% boost in fuel economy, which adds up big for fleets.

Other Fixes for the Air Problem

That scoop’s not the only thing fighting drag. You’ve probably seen those long panels running along the sides of trailers—side skirts. They block air from hitting the underside in the first place. Depending on where and how a truck runs, they can improve fuel efficiency by around 4–6%, according to Freight Wing.

But they don’t fix everything. A lot of drag still comes from the back end of the trailer—basically, the giant flat wall that acts like a parachute. Some say it’s responsible for a quarter of all drag on a truck.

The goal with all these fixes? Make hauling stuff cheaper. That’s also why most U.S. semis use inline-6 engines instead of gas-guzzling V8s.

And if you think this is just a truck problem, it’s not. Aerodynamic drag messes with fuel economy in every vehicle. Even your SUV. Especially your SUV. (A Toyota Land Cruiser has the aerodynamics of a loaf of bread.)

So yeah—that scoop under a semi? It’s not just plastic. It’s fuel money not going up in air.